Means for debossing and aperturing plastic webs are generally known in the art. U.S. Pat. No. 3,054,148 issued to Zimmerli on Sept. 18, 1962 discloses a process of producing a perforated thermoplastic sheet wherein a molding element rotates past a stationary drum having a fixed vacuum slot at a predetermined location on its periphery. The movable molding element is mounted around the surface of a stationary drum and is adapted to be rotated freely thereon. For purposes of rotating the molding element, a gear drive may be employed which is adapted to mesh with gearing provided on the element itself. As will be apparent from an inspection of the drawing figures in the Zimmerli patent, the aperture pattern in the molding element is relatively large, and the molding element illustrated in the drawing figures has sufficient rigidity and strength to permit its unsupported passage across a vacuum slot located at a predetermined point on the periphery of the stationary drum. Unfortunately, when fine-scale patterns involving three-dimensional plastic webs intended to provide the visual and tactile impression of fabrics are utilized, the forming structures necessary to produce such webs typically do not have sufficient rigidity or strength to permit their long term unsupported passage across a stationary vacuum slot. In addition, they often lack sufficient strength to permit long term transmission of the internal torsional stresses typically experienced when such forming structures are driven about a fixed drum at an elevated temperature.
Film forming methods and apparatus which appear to provide finer scale three-dimensional webs than those illustrated in the drawing figures of Zimmerli are also generally known in the prior art. For example, U.S. Pat. No. 2,776,451 issued to Chavannes on Jan. 8, 1957 discloses a moving perforate drum which passes across an internally located, fixed suction chamber secured at a predetermined point along the drum's periphery to impart a pattern to a heated thermoplastic film by the action of the suction. The forming structure layer(s) which contact the plastic material are preferably comprised of wire mesh. However, these forming structure layer(s) are supported about their entire periphery by means of a rigid perforate substrate which is fixed relative to the forming structure layer(s). Although the performate substrate provides the necessary rigidity and strength to move the forming structure layer(s) across the fixed suction chamber, fluid flow through the forming structure layer(s) is limited to that provided by the pattern of perforations in the rigid substrate. Thus, the application of a uniform fluid pressure differential to the plastic web is not achieved and the resulting appearance of the processed plastic web is non-uniform.
Generally similar approaches are disclosed in U.S. Pat. No. 4,155,693 issued to Raley on May 22, 1979, U.S. Pat. No. 4,157,237 issued to Raley on June 5, 1979, U.S. Pat. No. 4,351,784 issued to Thomas et al. on Sept. 28, 1982 and British patent application GB 2,021,479A in the name of Raley and Adams, published on Dec. 5, 1979. All of the foregoing references disclose cylindrical drums used to pattern and/or perforate a heated plastic film subjected to vacuum while supported on the drum. All of said drums have a film forming surface exhibiting whatever three-dimensional pattern of perforations is desired. As with Chavannes, the film contacting surface of the drum, which acts as the forming structure, is supported by a rigid perforate substrate to permit the suction provided by an internally located, fixed vacuum chamber to act on the innermost surface of a plastic web as the drum and the web traverse the vacuum chamber. Hence, a problem inherent in the system disclosed by Chavannes is also present in the aforementioned systems. Because the rigid perforate substrate utilized to support the forming structure does not move relative to the forming structure, it impedes the application of a uniform fluid pressure differential across the surface of the plastic web supported at the periphery of the drum during the drum's passage across the vacuum chamber. Where particularly fine-scale three-dimensional patterns of perforations are involved, this may result in complete blockage of fluid flow through the forming structure at points where no perforations are present in the supporting substrate. As a result, there is a failure to uniformly impart the three-dimensional pattern exhibited by the forming structure to the plastic web.
U.S. Pat. Re. No. 23,910 issued to Smith et al. on Dec. 14, 1954 discloses a slightly different version of a fluid-pervious cylindrical drum utilized to impart a three-dimensional pattern to plastic webs subjected to suction while supported thereon. As with the aforementioned references, the drum employs a rigid perforate substrate. However, the drum of Smith et al. employs a multiplicity of support ribs secured at spaced locations about its periphery to provide support to a flexible foraminous forming structure. Unlike the drums disclosed in the aforementioned prior art patents to Raley and Thomas et al. and the British application of Raley et al., the drum's rigid perforate substrate, which rotates the foraminous forming structure across an internally located, fixed suction box, does not directly contact the innermost surface of the foraminous forming structure. However, the support ribs which do contact the innermost surface of the forming structure can impede or eliminate fluid flow through the forming structure at their points of contact and thereby create unpatterned and/or unapertured areas at the points of contact between the foraminous forming structure and the support ribs.
Other prior art approaches to debossing and aperturing plastic webs have employed flexible forming structures in lieu of rigidly constructed or rigidly supported cylindrical drums. In one embodiment for producing patterned plastic webs of predetermined length, the aforementioned patent to Smith et al. discloses the use of a discrete length of flexible foraminous forming material which passes across a stationary planar suction box with a web of plastic supported on its uppermost surface. The discrete length of foraminous material is rewound at the discharge end of the forming operation and repositioned at the infeed to produce another discrete length of film having the pattern of the foraminous material therein to avoid the effect of a seam. During its traverse of the suction box, the foraminous material is supported across the width of the plastic web by a multiplicity of support rollers extending in the cross-machine direction. The edges of the foraminous material are sealed to the suction box by means of movable deckle straps which travel with the foraminous material during its passage across the suction box. As can be seen in FIG. 1, the flexible foraminous material is not continuously supported in the machine direction intermediate the support rollers as it traverses the suction box. Accordingly, deformation of the flexible foraminous material can occur intermediate the support rollers due to the lack of continuous machine direction support. This problem becomes more pronounced where wider webs, more flexible foraminous materials and/or higher vacuum levels are involved. As will be appreciated, such deformation and wrinkling in the forming structure results in wrinkling and objectionable appearance in the resultant patterned plastic webs.
Other approaches employing flexible forming structures are disclosed in U.S. Pat. No. 3,957,414 issued to Bussey, Jr. et al. on May 18, 1976 and U.S. Pat. No. 3,966,383 issued to Bussey, Jr. et al. on June 29, 1976. However, the structure generally disclosed in the patents to Bussey, Jr. et al. utilizes a pair of support rollers in conjunction with fixed underlying deckles to impart a pattern to a plastic web which passes across a vacuum box at an elevated temperature. As can be seen from the drawing figures, the deckles utilized to seal the sides of the vacuum box also support the outermost edges of the flexible forming structure as it passes across the vacuum box, thereby blocking fluid flow through the forming structure at points coinciding with the deckles. Furthermore, for wider web widths, more flexible forming structures and/or higher vacuum levels, the use of such supporting deckles at points intermediate the outermost edges of the web would likely be necessary to provide support to the flexible forming structure in order to prevent deformation during its passage across the suction box. The use of such intermediate deckles would, of course, result in blockage of fluid flow at points coinciding with the deckles, and hence unpatterned and unapertured areas in the resultant plastic webs.
One particular preferred prior art method for debossing and aperturing a running ribbon of thermoplastic film which substantially reduces the severity of the foregoing problem is generally disclosed in commonly assigned U.S. Pat. No. 4,151,240 issued to Lucas et al. on Apr. 24, 1979, said patent being hereby incorporated herein by reference. Briefly, the apparatus disclosed in the Lucas et al. patent comprises means for continuously converting a ribbon of thermoplastic film into a debossed and apertured film by directing hot air jets against one surface of the film while applying vacuum adjacent the opposite side of the film. The aforementioned operations are preferably carried out while maintaining sufficient control of the film to substantially obviate wrinkling and/or macroscopically distending the film. In a particularly preferred embodiment, the debossing and aperturing means include a rotatably mounted debossing/aperturing cylinder having closed ends, a nonrotating triplex vacuum manifold assembly and hot air jet means. The film contacting surface of the debossing/aperturing cylinder exhibits the three-dimensional pattern to be imparted to the plastic film.
In a particularly preferred embodiment of the Lucas et al. invention, the debossing/aperturing cylinder is constructed employing a tubular laminate forming structure of the type generally disclosed in commonly assigned U.S. Pat. No. 4,342,314 issued to Radel et al. on Aug. 3, 1982, said patent being hereby incorporated herein by reference. Forming structures of this type permit the production of debossed and apertured three-dimensional plastic webs having a predetermined, precisely regulated pattern of extremely fine scale. In particularly preferred embodiments, such webs can be made to appear fiber-like to the naked eye.
Unfortunately, when such finely patterned tubular forming structures are utilized on a cylindrical debossing/aperturing drum of the type generally disclosed in the aforementioned patent to Lucas et al., the radially oriented internal support members utilized to reinforce the tubular forming structure are likely to obstruct or totally block the fluid permeability of the forming structure at points of contact therebetween. Since there is no relative movement between the forming structure and the support members in use, the effectiveness of the fluid pressure differential, e.g., the vacuum, applied at the interior surface of said drum may be reduced or in some cases completely eliminated at such points of contact. Furthermore, the forming structure support members may actually contact and provide support to the film being processed, thereby tending to prevent rupture at such points. As a result, finely debossed and apertured films produced utilizing this technique typically remain unapertured wherever such a support member contacts the innermost surface of the tubular forming structure. This effect is most pronounced in situations where the overall thickness of the forming structure is relatively thin. As pointed out earlier herein, this is undesirable from both an aesthetic standpoint as well as from a functional standpoint in those situations where uniform fluid permeability is desired in the resultant three-dimensional plastic web.
The commonly assigned allowed patent application of William I. Mullane entitled METHOD AND APPARATUS FOR UNIFORMLY DEBOSSING AND APERTURING A RESILIENT PLASTIC WEB, Ser. No. 06/230,488, filed Feb. 2, 1981, issued on Apr. 10, 1984 as U.S. Pat. No. 4,441,952, and hereby incorporated herein by reference, discloses particularly preferred tubular forming structures constructed so as to alleviate the foregoing problems even further. In particular, such forming structures employ capillary networks which exhibits a cross-sectional area which reaches a minimum intermediate the outermost and innermost surfaces of the tubular forming structure and thereafter increases in the direction of the innermost surface of the forming structure to increase fluid permeability.
As with the structures of Radel et al., the tubular laminate forming structure of Mullane is preferably supported on a cylindrical drum of the type generally disclosed in the aforementioned patent to Lucas et al. by a multiplicity of radially oriented support members having lands which contact the innermost surface of the tubular member. Although the lands on the support members do not move relative to the innermost surface of the laminate forming structure, they are, in most instances, of insufficient cross-section to obstruct fluid flow through the capillary networks coinciding with their points of contact. Thus, the configuration of the individual capillary networks in the fluid-pervious laminate structure disclosed in the aforementioned patent application of Mullane permits substantially uniform application of a fluid pressure differential, usually vacuum, to the lowermost surface of the plastic material as the forming structure and the radially oriented support members traverse the vacuum chamber. In addition, the overall thickness of the Mullane structure prevents contact between the film and the support members prior to rupture. As a result, the heated plastic material is uniformly debossed and apertured substantially in conformance with the surface of the forming structure.
It will, of course, be appreciated by those skilled in the art that film forming structures of the type generally disclosed in the aforementioned application of Mullane are more complex, and hence more costly, than structures of the type generally disclosed in the aforementioned patents to Lucas et al. and Radel et al. Depending upon the useful life of such structures, this can significantly increase the cost of plastic webs produced thereon. It is further recognized that for webs involving extremely fine, closely spaced aperture patterns, even the invention disclosed in the aforementioned patent application of Mullane may not totally eliminate obstruction to fluid flow, since there is at least some contact between the innermost surface of the tubular forming structure and the radially oriented support members of the cylindrical drum.
Accordingly, it is an object of the present invention to provide method and apparatus for uniformly debossing and aperturing a resilient plastic web without limitation as to the fineness of scale of the aperture pattern exhibited by the forming structure.
It is another object of the present invention to provide method and apparatus for exposing the entire surface of a plastic web supported on a moving forming structure to a fluid pressure differential as the forming structure passes across a stationary fluid pressure differential zone.
It is another object of the present invention to provide means for driving said forming structure so that the torsional stress generated in said forming structure is minimized.
It is still a further object of the present invention to provide support means in the fluid pressure differential zone which are stationary relative to said traveling forming structure, yet which permit all portions of said forming structure to be subjected to a fluid pressure differential at some point during their traverse of said fluid pressure differential zone.
It is still another object of the present invention to provide support means which exert a constant force against the contacting surface of said forming structure as said forming structure passes across said fluid pressure differential zone, regardless of irregularities encountered during the processing operation.
It is still another object of the present invention to provide support means for said forming structure in said fluid pressure differential zone which will minimize wear and/or damage to the contacting surface of said forming structure and thereby greatly extend the life of said forming structure.
Yet another object of the present invention is to provide stationary support means in said fluid pressure differential zone which permit the use of either flexible or rigid forming structures.